Water-abrasive cutting systems are used for the cutting of various materials and objects. For example, they are used for cutting pipes, in order, for example to cut oil carrying pipes under the seabed. Such cutting systems can however also be used for cutting pipes in other applications, for example, refineries, well building etc. Furthermore, not only pipes but also objects having different geometries can be cut with these systems. For cutting pipes a cutting head which is connected via a hose to a high-pressure pump and an abrasive agent mixing unit, is inserted into the inside of the pipe as far as the height of the cut to be made. There the cutting device is then fixed with a fixing device inside the pipe and the pipe wall is cut through from inside using a high-pressure water jet to which abrasive agent is added. A problem with these cuts is that the work must be carried out without visual monitoring and therefore it is difficult to detect whether the pipe wall has been completely cut through this means in the radial direction and is completely cut through over the entire circumference. This is particularly difficult to detect since these pipes are usually configured as multi-shell, comprising a plurality of metal pipes lying inside one another, where the intermediate spaces of the metal pipes are filled with concrete. At the same time however the pipes are not always arranged in a centered manner and the intermediate spaces are not always completely filled so that no uniform cutting conditions are given.
An apparatus for cutting through pipes under water is known, for example, from DE 10 2011 052 399 A1. In this system it is provided to use a sensor, preferably a hydrophone, with the aid of which the passage of the water jet through the pipe wall is detected. In particular in the multi-shell pipes described however, a reliable detection of the cutting-through is not always ensured, which is in particular attributable to the fact that the filling of the intermediate spaces between the individual pipes and also the material which surrounds the pipe on the outer side is not known and depending on these states, different noises can be detected which do not always indicate a complete cutting-through of the pipe wall.